A novel "weak toxin" (WTX) from Naja kaouthia snake venom competes with [125 I]␣-bungarotoxin for binding to the membrane-bound Torpedo californica acetylcholine receptor (AChR), with an IC 50 of ϳ2.2 M. In this respect, it is ϳ300 times less potent than neurotoxin II from Naja oxiana and ␣-cobratoxin from N. kaouthia, representing short-type and long-type ␣-neurotoxins, respectively. WTX and ␣-cobratoxin displaced [125 I]␣-bungarotoxin from the Escherichia coli-expressed fusion protein containing the rat ␣7 AChR N-terminal domain 1-208 preceded by glutathione S-transferase with IC 50 values of 4.3 and 9.1 M, respectively, whereas for neurotoxin II the IC 50 value was >100 M. Micromolar concentrations of WTX inhibited acetylcholine-activated currents in Xenopus oocyte-expressed rat muscle AChR and human and rat ␣7 AChRs, inhibiting the latter most efficiently (IC 50 of ϳ8.3 M). Thus, a virtually nontoxic "three-fingered" protein WTX, although differing from ␣-neurotoxins by an additional disulfide in the N-terminal loop, can be classified as a weak ␣-neurotoxin. It differs from the short chain ␣-neurotoxins, which potently block the muscle-type but not the ␣7 AChRs, and is closer to the long ␣-neurotoxins, which have comparable potency against the above-mentioned AChR types.

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The characterisation of interspecies differences in gene regulation is crucial to understanding the molecular basis of phenotypic diversity and evolution. The atonal homologue Atoh7 participates in the ontogenesis of the vertebrate retina. Our study reveals how evolutionarily conserved, non-coding DNA sequences mediate both the conserved and the species-specific transcriptional features of the Atoh7 gene. In the mouse and chick retina, species-related variations in the chromatin-binding profiles of bHLH transcription factors correlate with distinct features of the Atoh7 promoters and underlie variations in the transcriptional rates of the Atoh7 genes. The different expression kinetics of the Atoh7 genes generate differences in the expression patterns of a set of genes that are regulated by Atoh7 in a dose-dependent manner, including those involved in neurite outgrowth and growth cone migration. In summary, we show how highly conserved regulatory elements are put to use in mediating non-conserved functions and creating interspecies neuronal diversity.

J. Neurochem. (2010) 113, 1307–1318. Abstract Retinal excitotoxicity is associated with retinal ischemia, and with glaucomatous and traumatic optic neuropathy. The present study investigates the role of c‐Jun N‐terminal kinase (JNK) activation in NMDA‐mediated retinal excitotoxicity and determines whether neuroprotection can be obtained with the JNK pathway inhibitor, d‐form of JNK‐inhibitor 1 (d‐JNKI‐1). Young adult rats received intravitreal injections of 20 nmol NMDA, which caused extensive neuronal death in the inner nuclear and ganglion cell layers. This excitotoxicity was associated with strong activation of calpain, as revealed by fodrin cleavage, and of JNK. The cell‐permeable peptide d‐JNKI‐1 was used to inhibit JNK. Within 40 min of its intravitreal injection, FITC‐labeled d‐JNKI‐1 spread through the retinal ganglion cell layer into the inner nuclear layer and interfered with the NMDA‐induced phosphorylation of JNK. Injections of unlabeled d‐JNKI‐1 gave unprecedentedly strong neuroprotection against cell death in both layers, lasting for at least 10 days. The NMDA‐induced calpain‐specific fodrin cleavage was likewise strongly inhibited by d‐JNKI‐1. Moreover the electroretinogram was partially preserved by d‐JNKI‐1. Thus, the JNK pathway is involved in NMDA‐mediated retinal excitotoxicity and JNK inhibition by d‐JNKI‐1 provides strong neuroprotection as shown morphologically, biochemically and physiologically.

The HES proteins are known Notch effectors and have long been recognized as important in inhibiting neuronal differentiation. However, the roles that they play in the specification of neuronal fate remain largely unknown. Here, we show that in the differentiating retinal epithelium, the proneural protein ATOH7 (ATH5) is required for the activation of the transcription of the Hes5.3 gene before the penultimate mitosis of progenitor cells. We further show that the HES5.3 protein slows down the cell-cycle progression of Atoh7-expressing cells, thereby establishing conditions for Atoh7 to reach a high level of expression in S phase and induce neuronal differentiation prior to the ultimate mitosis. Our study uncovers how a proneural protein recruits a protein known to be a component of the Notch signaling pathway in order to regulate the transition between an initial phase of selection among uncommitted progenitors and a later phase committing the selected progenitors to neuronal differentiation.